There are tens of thousands of sports fields in the United States, from high-profile professional fields in stadiums built for hundreds of millions of dollars to the recreational fields on which 4- and 5-year-olds play their first games.
Regardless of where it is and who’s playing, the expectations are the same: a quality playing surface that’s safe. While the selection of turfgrass species and cultivars is important, as are proper cultural practices, the foundation of a good sports field is the soil in which the grass is grown.
Soil serves many purposes on an athletic field, and the properties of the soil greatly affect its capacity to do so. Soil acts as a reservoir for water and nutrients. It also provides the media in which the grass plants are rooted. Ideally, your soil should provide adequate, but not excessive, water and nutrients. It also should also possess sufficient strength to keep the grass well-rooted, providing a safe and resilient surface.
Soil formation requires considerable time and is the result of many processes, including:
- Addition of water, dissolved minerals, and organic matter;
- Losses of water and minerals due to leaching or erosion;
- Movement of materials such as organic colloids, nutrients, and salts;
- Transformations such as clay and humus formation.
Soil components include the solids, which are composed of minerals and organic matter, liquid water, and air. A healthy soil for plant growth has an ideal ratio of solids (50 percent), liquids (25 percent), and gases (25 percent).
Soil particles are generally categorized by size into three categories: sand, silt and clay.
Sand has the largest particle size. Because of the larger particle size, there are larger spaces between particles, resulting in larger pores and more rapid movement of water both into the soil, referred to as infiltration, and out of the soil, referred to as percolation or drainage.
Clay is the opposite; it has very small particles, a large capacity to hold water and nutrients but also drains very slowly.
Native soil fields
In some parts of the country, you may be fortunate enough that the native soil is a loamy-sand that has good infiltration and drainage characteristics. However, the native soil, whether or not the topsoil has been removed, in many parts of the country tends to be heavy clay.
And, because they are the least expensive type to construct, the overwhelming majority of athletic fields are constructed on native soil. These fields are also referred to as natural soil, or soil-based fields.
Native soil fields that are properly managed and free of problems with compaction may actually have the best potential lateral shear strength and, therefore, the best potential to tolerate wear and lots of play.
More often than not, however, poor infiltration and drainage rates and compaction of soil are the big challenges for a sports field manager with a native soil field.
The long-term quality of an athletic field is often directly correlated to the ability of the field to drain excess water.
To attempt to compensate for poor drainage associated with clay and compaction, many native soil fields are regularly core-aerified and also crowned, so that the middle of the field is higher, and this may improve runoff of excess water from the field.
Even if the field is crowned, anyone who has watched a football game played in heavy rain knows that the problem of clay soil is that water doesn’t infiltrate or drain quickly enough. This problem is made worse when the turf thins as the playing season progresses.
What is perhaps less obvious is that, when a game is played under these conditions, the resulting deterioration of the soil structure can be devastating to subsequent turfgrass growth and development.
Given that compaction and deterioration of soil structure are such severe problems for athletic turf managed on native soil, sand might seem like the obvious solution. However, while less likely to compact and better for drainage, sand is actually not an ideal medium for growing grass.
The rapid drainage comes at the expense of its ability to hold water, meaning the surface will dry out much more readily and is more susceptible to droughty conditions.
Also, sand has a lower capacity to hold and exchange nutrients, measured by cation exchange capacity (CEC) on a soil test. Because of this, a sandy soil should be tested more often and fertilized more frequently with lower amounts of nutrients.
If your native-soil field has problems caused by inadequate drainage, and you’re willing and able to adjust to the management needs of growing grass on a sand-based medium, then the solution may be to change the soil profile.
Athletic fields tend to be broadly categorized into three types based on the properties of the soil. In addition to native soil fields, which were just discussed, there are constructed fields and modified-soil fields.
Constructed soil fields
The second type of field is the constructed, or soil-less field, and it offers many advantages. The most significant advantage is the ability to drain rapidly. They are the most expensive to construct, but also have the greatest potential to perform in extreme conditions.
You will not, however, necessarily get more games out of a sand-based field, due to the lower relative shear strength compared with native soil. Maintaining a stand of grass on sand is actually more of an agronomic challenge than maintaining it on native soil. Soil-less fields are droughtier because the macropores in sand hold less water than the micropores of clay, meaning that an irrigation system is an absolute requirement.
While agronomic management may be more difficult, constructed soil fields offer the distinct advantage of rapid drainage, which is critical when sports are played during rain.
But, the cost of removing the top 12 to 16 inches of soil and installing drain tile, a gravel layer and a 12-inch, sand-based root zone can be cost-prohibitive for most operations.
Modified soil fields
In the middle of the spectrum between native soil fields and constructed soil fields are modified soil fields. These are native soil-based fields that are modified with the addition of sand.
The type of sand and its physical characteristics are very important to consider (simply put, not all sands are equal). For example, you’ll want to avoid sand that has high gravel content. What may be difficult to understand is that, if not modified with enough sand, neither drainage nor resistance to soil structure degradation will improve. The reason is that sand has a much larger particle size than clay, which results in larger spaces between the particles.
Proper sand selection is critical, as are the methods by which the sand is added to the soil profile. If done during field construction, the amount of sand added should be based on the characteristics of the native soil present.
In order to avoid the problems caused by adding small amounts of sand and mixing it with clay, a different strategy should be employed when attempting to modify an existing field with sand. After core aerification and removal of the soil cores, sand alone or sand mixed with organic matter can be added.
The sand that fills in the aerification holes provides zones that are more ideal for air and water movement and also grass rooting.
If core aerification and topdressing are performed aggressively over a period of years, the root zone of the field can be significantly improved, as can infiltration and drainage rates. This produces a distinct layer or cap of sand on top of the native soil.
Removal of the cores following aerification is a must in order to produce a layer of sand on top of the native soil. Otherwise you’ll be mixing the sand with the native soil. Again, the goal is to produce a distinct layer of sand on top of the native soil surface, much in the same way that golf course superintendents have produced push-up-style greens on golf courses.
Modifying a native soil field is usually expensive, not only because of the cost of the material used to amend the soil, but also because modified fields require the installation of an irrigation system. However, there are methods of modifying a field that are more economical and, in all cases, a modified field costs less than a constructed field.
If done properly, modifying a field produces noticeable results pretty quickly, which makes it easier to obtain funding from the school, park district, etc., in order to continue the modification program.
Clay associated with native soil has many small particles that may be bad for drainage. But clay is very good at holding nutrients. As a result, when fertilizing a native soil field, you may be able to get away with three or four rounds of fertilizer per year. By contrast, sand fields need to be fertilized more often but with a lower amount of fertilizer.
While modified fields have improved drainage characteristics, they, along with native soil fields, usually will require a crown. A crowned field is acceptable for football, but it’s not ideal for sports such as soccer. Also, both native and modified fields benefit from a trench or catch basin along the perimeter of the field to remove runoff water from the field. Without this, the sideline will be excessively wet.
So, modified fields may not completely solve drainage problems the way that a constructed soil field would, but the reduced cost makes a modified field more practical for most budgets.